Replacing an ink cartridge (or a pencil cartridge) in a retractable writing instrument having push-to-extend, push-to-retract operating mechanism typically involves unscrewing the two parts forming the body of the pen for access to the cartridge enclosed therein. A number of small components, such as a plunger, rotary ratchet and a helical compression spring, typically come loose or must be removed during this operation. The compression spring may be mounted about the cartridge, with opposing ends abutting a stop on the cartridge and the body respectively, requiring it to be placed about the replacement cartridge, before the rest of the components are carefully re-installed. U.S. Pat. No. 4,991,988 describes a pen with these features. It will therefore be understood that there is a need for an improved retractable writing instrument of this type, which allows users to more readily replace the cartridge without the risk of releasing small easily-lost components. It is an object of the present invention address this need or more generally to provide an improved retractable writing instrument.